How to Choose Actuator for Humanoid Robot: Joint-by-Joint Selection Criteria

Peak torque range: 80-150 Nm (walking), 200-300 Nm (running/jumping). The hip must swing the entire leg mass (~15-20% of body weight) through a large range of motion. Speed requirement: 3-6 rad/s for walking gait. The yaw axis requires the lowest torque but adds significant complexity for multi-DoF integration.

Peak torque range: 100-200 Nm (walking), 250-400 Nm (running). The knee experiences the highest dynamic loads during stance phase —up to 3-5x body weight during running. High shock load tolerance is critical. Speed: 4-8 rad/s. Regenerative braking capability is valuable for energy recovery in cyclic motion.

Peak torque range: 40-80 Nm (walking), 100-150 Nm (running). The ankle requires fast response for ground adaptation and balance. Speed: 3-5 rad/s. Backdrivability is important for passive compliance during stance. Ankle actuators must tolerate off-axis loads from ground contact.

Peak torque range: 30-60 Nm (arm manipulation), 80-120 Nm (with payload). The shoulder requires the widest range of motion in the upper body. Speed: 4-6 rad/s for natural arm motion. Gravity compensation is a design consideration for sustained positioning.

Peak torque range: 15-30 Nm (unloaded), 40-60 Nm (with 5 kg payload). Speed: 5-8 rad/s. The elbow has a more constrained space envelope —actuator diameter is often the limiting factor. High speed-to-torque ratio is desirable for quick manipulation tasks.

Peak torque range: 5-15 Nm. Speed: 6-10 rad/s. The wrist demands the smallest, lightest actuator in the humanoid. High reduction ratio in a compact form factor. Backdrivability is highly desirable for compliant manipulation and force-sensitive tasks.

KV determines the motor's speed at a given bus voltage. For a 48 V system targeting 80 RPM output through a 50:1 reducer: motor speed = 80 × 50 = 4,000 RPM, so KV = 4,000 / 48 ≈ 83 RPM/V. Higher KV reduces torque constant (Kt), so choose the lowest KV that meets your speed target to maximize torque output.

The motor's rated torque (continuous) must cover the RMS torque of the duty cycle, while peak torque (typically 2-3x rated) must cover transient loads. Humanoid walking cycles typically have a 2-3:1 peak-to-RMS ratio. Select a motor where rated torque × gear ratio ≥ required joint torque / 1.5.

Higher speed motors produce more iron losses and heat. For humanoid joints, target motor speed in the 3,000-6,000 RPM range. Validate that winding temperature stays below 100°C under the worst-case duty cycle. Copper fill factor and slot insulation class (minimum Class F, 155°C) are critical reliability factors.

Map the joint torque profile over one complete gait cycle (typically 0.8-1.2 seconds for walking). Calculate τ_RMS = sqrt((Στ_i² × t_i) / T_total). Compare this against the actuator's continuous torque rating. For humanoid walking, RMS torque is typically 40-60% of peak torque.

The actuator's thermal time constant determines how quickly it heats up under load. For humanoid joints, a minimum thermal time constant of 15-20 minutes is recommended to handle the cyclic loading of walking without thermal runaway. Metal housing designs (aluminum or stainless steel) dissipate heat significantly better than plastic.

Always benchmark actuator temperature rise under the actual duty cycle —not just the rated torque spec. A 48 V actuator rated for 5 Nm continuous may reach 120°C in 8 minutes if the duty cycle includes frequent peak torque bursts. Request thermal test data from your supplier.

Many teams calculate actuator torque requirements using static weight distribution alone, ignoring dynamic amplification factors. During walking, ground reaction forces amplify joint torques by 2-5x. During running or jumping, this can reach 8-10x body weight. Always apply a DAF of at least 2.0 for walking-capable humanoids and 4.0 for running-capable designs.

Harmonic drives excel at precision but have limited shock load capacity (typically 2-3x rated torque). In knee and hip joints where ground impact can transmit 5x rated torque in milliseconds, planetary gearboxes or QDD designs provide the necessary shock robustness. Use a hybrid approach: harmonic for upper body and ankle, planetary/QDD for hip and knee.

An actuator rated for 100 Nm continuous may overheat in 5 minutes under the pulsed loading pattern of humanoid walking. The RMS torque over a gait cycle, not the peak torque, determines whether the actuator can sustain operation. Request thermal rise curves from your supplier and test under your actual duty cycle before committing to a design.

Humanoid locomotion relies on torque-controlled walking —not just position control. Low-resolution encoders cause torque ripple and limit cycle oscillations. Choose actuators with at least 17-bit (131,072 counts/rev) output-side resolution for lower body joints and 19-bit for precision upper body manipulation. ZHR-H series includes high-resolution absolute encoders as standard.

Coordinating 20+ joint actuators on a humanoid requires deterministic, low-latency communication. A 2 ms CANopen bus cycle limits control bandwidth to 500 Hz —barely adequate for stable walking. For humanoids with 12+ DoF, use EtherCAT or at minimum CAN FD to achieve >1 kHz joint control loops. ZHR-P series supports EtherCAT at 2,000 Hz PDO cycle with <5 µs jitter.

For a 50-70 kg humanoid, the hip pitch joint typically requires 80-150 Nm for walking and 200-300 Nm for running/jumping. The hip roll requires 100-180 Nm. These values depend on your robot's weight distribution, leg length, and intended gait speed. Always calculate using your specific robot parameters and apply a dynamic amplification factor of at least 2.0× for walking and 4.0× for running.

A hybrid approach is recommended. Use planetary gearboxes or QDD for hip pitch and knee joints where shock loads from ground impact are highest. Use harmonic drives for ankle pitch, hip roll, and all upper body joints where precision and zero backlash are critical. This combination optimizes both shock tolerance and positioning accuracy while managing overall cost.

48 V is the most common choice for mid-size humanoids (50-80 kg), offering a good balance of power density and safety. 72 V systems provide higher peak power for running-capable robots but require more careful electrical isolation. 24 V is only suitable for very small humanoids under 30 kg. Higher voltage reduces current for the same power, enabling smaller gauge wiring and lower I²R losses.

Step 1: Calculate the RMS torque of your walking cycle. Step 2: Compare against the actuator's continuous torque rating at the ambient temperature of your operating environment. Step 3: Request thermal rise curves from the manufacturer. Step 4: Test the actuator under a representative duty cycle for at least 30 minutes while monitoring winding temperature. If temperature stabilizes below 100°C (for Class F insulation), the actuator is thermally suitable.

Yes, absolutely. ZHR-H (harmonic) and ZHR-P (planetary) series share the same communication protocol interface (EtherCAT/CANopen), enabling seamless integration in a single robot. Many humanoid developers use ZHR-P for high-torque lower body joints and ZHR-H for precision upper body joints. Both series support 48 V and 72 V bus voltages and can be controlled from the same master controller.

For humanoids with 12+ joints, EtherCAT is the recommended choice, offering 125 µs cycle times with <1 µs jitter across all axes. This enables torque-controlled walking at 1-2 kHz loop rates. For smaller systems with 6-8 joints, CAN FD provides a cost-effective alternative at up to 2,000 Hz cycle time. See our detailed comparison: EtherCAT vs CANopen for Robot Joints.